The Mode Select Beacon System (the Mode S system) is a secondary surveillance radar system using monopulse technology to provide higher positional accuracy than the current Air Traffic Control Radar Beacon Systems (ATCRBS). Mode S also provides discrete addressing and a bidirectional digital data link.
The Mode Select Beacon System (Mode S) is a radar beacon air traffic control system, in which coded radar pulses are sent from radar antennas on the ground and received by beacon transponders mounted in aircraft; these beacons then send coded response pulses back to the ground stations, for the purpose of aircraft tracking and communications. These same beacon transponders can also be used to send coded pulses to beacon transponders in other aircraft (air to air, as opposed to air to ground), for the purpose of aircraft collision avoidance, a system known as TCAS. The Mode S beacon transponders are also compatible with an older radar beacon system known as Air Traffic Control Radar Beacon Systems (ATCRBS).
A new use for the Mode S beacon transponders has been proposed, known variously as ADS-Mode S and GPS-Mode S. This would entail having the transponders emit additional coded pulses, called GPS squitter pulses, to enable the ground air traffic controllers to have GPS (Global Positioning System) positional information on the aircraft location, which is based on highly accurate navigational satellite data.
A potential problem with such a proposed system is that all of these coded pulses are being transmitted on the same radio frequency. Thus, the more pulses that are being emitted, the greater chance that pulses transmitted from one aircraft will interfere with a transmission from another aircraft, potentially causing both messages to be missed by the intended receiver. Before approving a plan that calls for more transmissions, the question must be answered, how many aircraft can co-exist in a given region of the sky before interference effects cause ADS-Mode S operation to fall below acceptable levels.
In G. Knittel and V. Orlando, "ADS-Mode S" in 38th Annual Air Traffic Control Association Conference Proceedings, pages 230-236 (ATCA, 1993), the authors considered reply interference effects on ADS-Mode S squitter and developed estimates of the maximum number of aircraft which can be handled by an ADS ground station, as a function of ATCRBS and Mode S reply levels. These estimates were made analytically, which required several explicit and implicit assumptions to make the problem analytically tractable. For example, this paper assumed that the arrival at the ADS-Mode S antenna of ATCRBS replies from many aircraft, and the probability of interference from Mode S replies, can be treated as a Poisson process. This resulted in a conservative estimate of capacity.
ADS-Mode S is an Automatic Dependent Surveillance (ADS) concept in which aircraft would transmit Global Positioning System (GPS) estimates of their position to ground stations by using the squitter capabilities of the Mode S beacon transponder. A squitter is a random, as opposed to a scheduled, transmission of data. Such a system is considered in the above-identified paper by Knittel and Orlando, in which GPS equipped aircraft would emit a long Mode S squitter containing positional information twice a second. A limiting factor in such a system is that a squitter arriving randomly at the receiving ground antenna may be destructively interfered with by replies from other transponders on the common 1090 MHz frequency. Thus, there will be some critical density of aircraft and reply rates which will cause the probability of receiving squitter positional updates to fall below an acceptable minimum.
The Knittel and Orlando paper chooses a 99.5% probability of receiving at least one ADS squitter update from an airplane every five seconds as their minimum acceptable ADS criteria. That paper then uses analytic methods to determine the maximum number of aircraft which can be handled by an ADS ground station for various reply rates, based on interference limitations.
The estimates of interference effects in the Knittel and Orlando paper are made analytically, which requires several explicit and implicit assumptions to make the problem analytically tractable. The major assumption in this analytic estimate is that the arrival of ATCRBS replies from any aircraft at the ADS-Mode S receiver antenna, and interference from Mode S replies, can each be treated as a Poisson process. This means, among other things, that the replies' arrivals are uncorrelated with one another; that the rate of reply arrivals is steady over time; and that the probability of a reply arriving at any one given instant is the same as for any other instant. Also, since the Poisson distribution is a one parameter distribution, only the total number of replies in a second is considered relevant, i.e. 10 airplanes with 150 replies per airplane per second is treated the same as you would 100 airplanes with 15 replies per airplane per second.